Wafer inspection systems help a semiconductor manufacturer increase and maintain integrated circuit (IC) chip yields by detecting defects that occur during the manufacturing process. One purpose of inspection systems is to monitor whether a manufacturing process meets specifications. The inspection system indicates the problem and/or the source of the problem if the manufacturing process is outside the scope of established norms, which the semiconductor manufacturer can then address.
Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions are shrinking while wafer size is increasing. Economics is driving the industry to decrease the time for achieving high-yield, high-value production. Thus, minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for the semiconductor manufacturer.
Defects can be detected by comparing an image of a wafer to a reference image. However, different layers in a semiconductor wafer have different thicknesses. Even the same layer of a semiconductor wafer can have different thicknesses. Thickness variation can affect the gray level of an image because layer thickness affects reflectivity. Die-to-die material thickness variation can result in a different reflectivity between two of the dies, which leads to a different background gray level value for the images of the dies. This can be referred to as process variation.
Color variation or noise is an example of process variation. It is difficult to correct color variation or noise. This can make it impossible to identify the best mode for defect inspection because the defect cannot be seen in the color noise.
Process variation can make it difficult to find defects and may result in false positives. Increasing tolerance in a histogram can result in loss of some of the defects. Therefore, what is needed is an improved image processing technique.